In the related art, there has been known an air intake apparatus including a distribution passage through which external gas is distributed to a plurality of cylinders (for example, JP 2006-241992A (Reference 1)).
In above-described Reference 1, an air intake manifold (air intake apparatus) for guiding air to four consecutive cylinders of an engine is disclosed. The air intake manifold disclosed in Reference 1 includes four branch pipes (air intake passage) and a gas passage (distribution passage) for returning blow-by gas (external gas) to the engine. The gas passage is formed so as to branch stepwise (1→2→4) and extend from a gas introduction port into which the blow-by gas is introduced to the four branch pipes. That is, the gas passage is formed in a so-called tournament branch shape. Therefore, the air intake manifold is configured such that the blow-by gas is distributed to each of the four branch pipes and cylinders.
However, the inventor of the present application has found a problem that variation occurs in the distribution of the blow-by gas to the four consecutive cylinders depending on operating conditions of the engine even in a case where the gas passage is formed in the tournament branch shape in the air intake manifold disclosed in Reference 1. Therefore, in the gas passage formed in the tournament branch shape, it is desired to provide an air intake manifold capable of effectively suppressing occurrence of the variation in the distribution of the blow-by gas to the four consecutive cylinders.
Thus, a need exists for an air intake apparatus which is not susceptible to the drawback mentioned above.